Recently, Radio Frequency IDentification (RFID) systems have been widely used. Typically, some RFID systems utilize electromagnetic waves of a UHF band (900 MHz band) or a microwave band (2.45 GHz) as a communication medium, and some RFID systems utilize a magnetic field generated by mutual induction. Among them, the REID system utilizing an electromagnetic wave in the UHF band is attracting attention since the RFID system can provide relatively long communication distance.
A microstrip antenna is proposed as an antenna which is used by a reader-writer that communicates with an RFID tag utilizing an electromagnetic wave in the UHF band. The microstrip antenna includes a microstripline as the antenna (see Patent Reference 1 and Non-Patent References 1 and 2, for example).
There is a system which includes an antenna provided on a surface of a shelf. Merchandise to which an RFID tag is attached is arranged on the shelf. The system identifies that the merchandise is taken away from the shelf when the system becomes unable to detect the RFID tag. In such a system, it is preferable to use an antenna apparatus which can read the RFID tags attached to the merchandise provided in an area close to the surface of the antenna and can read the RFID tags over the entire surface of the shelf.
However, a communication distance of the conventional antenna is not sufficient and it is difficult to generate a uniform electric field over the entire surface of the antenna, particularly when size of the antenna becomes larger. Accordingly, it is difficult for the conventional antenna to provide uniform and sufficient communication distance.
Therefore, it is difficult to read all of the RFID tags uniformly in a case where a plurality of merchandise to which the RFID tags are attached is arranged on the shelf, in a case where the conventional antenna is used in the system as described above.